Fractures of normal bone usually result from injuries which frequently also involve surrounding tissues. Not all fractures can be treated conservatively as in number of instances treatment by immobilization, traction and other non-surgical methods results in non-unions, delayed unions and malunions. Prolonged immobilization can also lead to stiffness and degeneration of adjacent joints. For these reasons surgical treatment of fractures is often resorted to Anatomical reduction and bone immobilization by internal fixation with plates and screws. This is highly successful, but it too has its limitations not the least of which is stress-shielding. Anatomical reduction and compression of the fracture site with rigid internal fixation are generally achieved by the use of metallic dynamic compression sliding plates and screws. The plates are equipped with elongated holes with angulated sides. This allows bone to move independently of the plates. The holes in the bone plate are configured so as to allow the plate to glide in response to the pressure applied by the screw head and to push the bone fragments to achieve compression of bone fragments against each other. Such fixation usually produces good clinical results. A relatively recent addition to the internal rigid fracture fixation was the advent of locking plates. With conventional plates, screws can move independently from the plate, they may loosen with time. Premature screw loosening produces fracture instability, loss of fracture gap and non-union. Plate to bone compression and friction from the movement of the plate may negatively impact on the blood supply essential to fracture healing and formation of fracture callus, both external and internal. Osteoporotic bone may not be able to withstand high screw torque required to maintain screw-compression plate construct. With locking plates, the screws and plates form a single complex reducing the possibility of hardware failure since the motion between, screws, bone and plate is eliminated. The plate screw assembly distributes stress along the entire length of the plate, thus making it more suited for osteopenic bone and multi segment fractures. Locking screws are not better or worse than conventional screws. They are different. Locking screws use the screw pullout strength more effectively, but cannot compress fractures as conventional screws can. Hybrid plate fixation provides advantages of both techniques. Bridge plating can be performed with conventional, locked or hybrid fixation. The longer the bridge or comminuted segment, the more empty holes in currently available plates. Empty holes can double or triple the plate stresses by concentrating the stress which reduces fatigue life of the plate.
It is well understood that bone healing can also occur as a result of flexible osteosynthesis which provides only relative stability. This method of bone fixation is typically employed in comminuted fractures. The goal of this technique is to maintain length of the bone, its angulation and keep the end portions of fractured bones or joints in correct position without directly lifting or manipulating the fracture zone itself. This can be achieved with locking screws, which as we said do not have the capacity to compress bone fragments.
As is well known to those skilled in the art the bones where fractures occur as well as the types of the fractures vary greatly. Many fractures do not respond well to only rigid internal fixation with compression alone or flexible osteosynthesis with angular stability alone. This precipitated the development of bone plate assemblies which incorporate screws that allow compression or lock into the threaded plate hole. These have been the subject to several patents and publications known to those skilled in the art.
U.S. Pat. No. 8,632,545 B2 discloses the invention relating to a bone plate with a combination hole which can receive a bone screw that can be used to fix bone fragments and also be securely locked in a relationship of angular and axis stability with the bone plate. The invention discloses a bone plate system wherein the bone plate comprises a plurality of holes at least one of which is a combination hole that can receive screw capable of either locking or compressing the plate.
U.S. Pat. No. 6,669,701 discloses a bone plate with at least one hole with a substantially circular outer periphery and a second portion defining an elongated outer periphery, with threads extending over an angle of 100 to 270 degrees at the upper surface of the bone plate and 180 to 230 degrees at the lower surface of the bone plate. The bone plate as disclosed in this patent has only a partial thread in the hole which does not encompass the diameter of the hole completely.
U.S. Pat. No. 8,226,693 reveals an orthopaedic bone bridge for internally fixing and stabilizing fractured bones. It includes first and second bone plates attached to bone by screws. Plates are placed on the opposite sides of the fractures. The plates are mounted on a pair of elongated hollow legs on which the plates are mounted. The second plate is slidably engaged and is movable with respect to the first plate. A cable is attached to the first plate and extends through the legs and around the second plate. The cable provides tensile form between the plates when they are pulled apart thus compressing the fractured bone fragments by forming a bone bridge.
U.S. Pat. No. 5,057,111 reveals non-stress shielding bone healing device which can be with screws, pins or nails for attaching the device to the bone. At least one of two or more openings in the plate is a relaxation opening shaped in part as a truncated spherical section. The plate is preferably a polymer member formed of viscoelastic or resorbable materials.
It is well understood that bone screws used in bone plate assemblies to compress bone fragments also compress the bone plate to underlying bone. This result in stress shielding and undesirable osteopenia or osteoporosis in the bone beneath the plate. The second undesirable effect of compression plates which span the fracture site is the inhibition of the formation of the external callus with consequent weakening of the fracture site.
Bearing these facts in mind the prior art reveals the need for an improved and radically different system of bone plate assemblies.